The baking (curing) of organic films is critical to the manufacturing process used for all integrated circuits. This process is called “post apply bake” or PAB. Typical films include topcoat barrier layers (TC), topcoat antireflective layers (TARC), bottom antireflective layers (BARC), imaging layers (PR or photoresist) and sacrificial and barrier layers (hard mask) for etch stopping.
The bake process time and temperature are used to drive out solvents and cure or harden the film and thereby define the characteristics of the film at exposure and post exposure develop where the circuit feature is defined, prior to etching the feature into the substrate. By-products of the bake process are sublimation products (solids) and out-gassing (liquid) materials. These byproducts can build up on the interior of the bake chamber and in the exhaust lines causing defects to fall onto the wafer in process. Typically, a cleaning of the bake system is required when these byproducts build up to certain levels. These methods may not be sensitive enough in future designs to meet sub 32 nm processing requirements. Minimizing defects during wafer processing will continue to be a critical path to attaining cost effective manufacturing of advanced semiconductor devices. Hard particles can block etch processes causing electrical “open” or “short” in the circuit. In lesser size and if lucky with the location on the device, the hard particle may only create fatal perturbations in the active features' critical dimension (line/space or contact hole)
One source for hard particle defect generation in the imaging process is the post application bake process. In these processes, solvent rich, polymer containing spun on films are baked at temperatures close to and even well above the boiling point (temperature) of the casting solvent used. Temperatures range from 80 to 250 C. The most common byproduct of the post apply bake is volatile organic compounds (VOC's) and in some amount of polymer or other “hard” material that deposits on the inside of the bake chamber. This material builds up and will start to shed or peel off creating particles with time, usually within 4,000 to 15,000 wafers processed. The degree of particles generated depends on chemistry and bake temperature. The lid is of course mounted directly above the production wafer so anything that comes loose will fall onto the production wafer.
Current solutions for this is to cool down the bake system, remove the bake plate lid and clean it in various chemicals or dunk it in a bath of solvent. Then reassemble the bake system and test for process compliance. This can take 4 to 6 hours during which time the tool is unusable for manufacturing.
It is Anticipated More Frequent Cleaning of Bake Plate Lids Will be Required to meet the future device defect densities.